Muon spin relaxation studies of extremely concentrated paramagnetic electrolyte solutions

The diamagnetic signal in aqueous solutions of non-reacting salts arises from muons substituted for protons in water molecules. For concentrated solutions of paramagnetic salts, muon spin relaxation results from the strong dipolar and scalar interactions between the paramagnetic ion and the muon. Information on dynamic and structural aspects of such solutions is derived from measurements of the transverse relaxation rate as a function of applied field over a wide range. Data are presented for concentrated solutions of Mn(NO₃)₂ and solutions of composition Mn _x Ca_(1–x (NO₃)₂·6H₂O.     

Infrared intensities of ion-water interactions in aqueous electrolyte solutions

The i.r. absorption intensities have been measured of the fundamental vibrations of water molecules in metal halide aqueous solutions. Pseudo-isosbestic points were observed in the regions of both the stretching and bending vibrations and interpreted in terms of two states of water molecules in the solutions. The intensities of the stretching vibrations of water molecules coordinated to the dissolved ions have been estimated. The intensities become stronger in the order, Li⁺ > Na⁺ > K⁺ for the cations and F^- > Cl^- > Br^- > I^- for the anions. These orders are consistent with the strengths of the ion-water interactions, which are related to the dynamical behavior of the water molecules in the primary hydration shell.     

15N chemical shifts in concentrated aqueous solutions of ammonium salts

¹⁵N n.m.r. spectra have been recorded from salts of ¹⁵N enriched ammonium ions in aqueous solution. Shifts of the resonance frequency have been attributed to direct interaction between the ammonium and its counter ion in solution.     

X-Ray diffraction investigations of concentrated aqueous solutions of calcium halides

Concentrated aqueous solutions of calcium chloride and calcium bromide have been investigated by X-ray diffraction. In the diffraction patterns of aqueous chloride solutions of different concentrations, a maximum of intensity is observed in the region 0.6–1.1 Å⁻¹, while in the investigated aqueous bromide solution no maximum is observed in that region. This pre-peak suggests the existence of positional correlations in solution beyond direct contact. The interpretation is similar to the one proposed by the authors in previous investigations, and is supported by the calculations based on ad hoc molecular models. The influence of the anion is discussed.     

1H NMR study of water relaxation and self-diffusion in human serum albumin aqueous solutions

Summary Water proton spin-lattice relaxation and self-diffusion in aqueous solutions of human serum albumin have been studied by¹H NMR as a function of the protein concentration. Spin-lattice relaxation data, which display a nonlinear behaviour with the protein concentration, could be fitted with a two-phase model taking into account the experimentally determined hydration (?bound?) water values. Despite a similar trend is registered for the water self-diffusion coefficient, such a model has been found unable to explain the related experimental data taken as a function of the biomolecule concentration. On the other hand, the solute-induced proton self-diffusion decrease could be satisfactorily interpreted by postulating an enhanced probability of hydrogen-bond formation occurring within the ?vicinal? water surrounding the biomolecules for several hydration shells. The consistency within the two models is discussed in connection with the magnetic interactions occurring within the solute-solvent systems.     

Effects of ion atmosphere on hydrogen-bond dynamics in aqueous electrolyte solutions

We have performed a series of molecular dynamics simulations of aqueous NaCl and KCl solutions at different concentrations to investigate the effects of ion atmosphere on the dynamics of water-water hydrogen bonds at room temperature. The average number of hydrogen bonds per water molecule decreases with increase of ion concentration. The dynamics of hydrogen-bond breaking is found to accelerate somewhat and that of hydrogen-bond structural relaxation, which occurs at a longer time scale, is found to slow down with increasing ion concentration for both NaCl and KCl solutions.     

Visualization of pressure-shift freezing and thawing of concentrated aqueous sucrose solutions

A visualization of pressure-shift freezing of 0.7 w/w sucrose solutions was carried out at three temperatures 268, 253 and 235 K by release of pressure from 200 MPa to atmospheric value. Furthermore, pressure-shift freezing at 268 K and additionally pressure-shift thawing was carried out for a solution of 0.2 sucrose mass fraction. The solid phase observed at 268 K in the case of solution with 0.2 w/w sucrose fraction was ice I. The phase changes of 0.7 w/w sucrose solutions at 253 K and 235 K resulted in formation of spherical solids which are hypothesized to be eutectics, i.e. crystals with structure containing sucrose hydrates and water. The visual evaluation revealed differences in size and distribution of spherical crystals. The solids formed at 235 K were more numerous, more homogenously distributed and finally smaller than those observed at 253 K. It is explained by the higher supercooling of the solution in the former case, which provided higher driving force for nucleation and crystal growth.     

A simple formula for the short-time self-diffusion coefficient in concentrated suspensions

A mean-field type expression for the short-time self-diffusion coefficient is proposed and compared to recent experimental data.     

pH dependence of electrokinetic behavior of dolomite and magnesite in aqueous electrolyte solutions

In this paper, electrokinetic potential and isoelectric point of dolomite (CaMg(CO₃)₂) and magnesite (MgCO₃) were determined. The effect of various ions such as Mg²⁺, Ca²⁺, Na⁺ and CO₃²⁻ on surface properties of dolomite and magnesite were also examined. Isoelectric points of dolomite and magnesite were determined as 6.3 and 6.8, respectively, in the absence of any electrolyte. H⁺ and OH⁻ ions are the potential determining ions of magnesite and dolomite, as predicted by electrokinetic potential studies.     

A mechanism of energy transfer in binuclear fluoride lanthanide complexes in aqueous electrolyte solutions

The energy transfer between different pairs of lanthanide ions bonded by fluoride bridges into labile binuclear complexes is studied in aqueous solution at different ratios between the concentrations of lanthanide ions and fluoride anions ([Ln]: [F]). It is shown that, if the concentrations [Ln] and [F] are of the same order of magnitude, the energy transfer rate constant k _t does not depend on the choice of the pairs of interacting ions and is determined by the association rate constant of Ln(III) ions into binuclear complexes. If the concentration of the lanthanide ions is much greater than that of the fluoride ions, k _t varies proportionally to the monomolecular energy transfer rate constants in the binuclear complexes. It is assumed that, in the first case, Ln(III) ions are bonded through two fluoride anions, whereas, in the second case, they are bonded through one anion. The analysis of the variations in k _t in the latter systems shows that the exchange-resonance mechanism should be taken into account for the explanation of the experimental data. The effects that the introduction into the solution of different contents of salts of strong acids—AlCl₃, MgCl₂, Ca(NO₃)₂, CsCl, RbBr, and NaCl—have on k _t and on the regularities of the energy transfer between Ln(III) ions bonded into binuclear fluoride complexes are studied. The effects of these electrolytes on the luminescence intensity and spectrum of Eu(III) ions and on the values of k _t for the energy transfer between Ln(III) ions bonded into binuclear complexes are analyzed. It is shown that, at some concentration ratio [Ln]: [F], for all electrolytes studied except AlCl₃, the value of k _t increases despite the fact that the concentration of mononuclear complexes of Ln(III) ions with fluorine decreases in the presence of these electrolytes. It is ascertained that the anions of strong acids in the outer sphere of lanthanide ions increase the association constant of Ln(III) ions in binuclear fluoride complexes.     

Hydration of the chloride ion in concentrated aqueous solutions using neutron scattering and molecular dynamics

Neutron scattering experiments were performed on 6 m LiCl solutions in order to obtain the solvation structure around the chloride ion. Molecular dynamics simulations on systems mirroring the concentrated electrolyte conditions of the experiment were carried out with a variety of chloride force-fields. In each case the simulations were run with both full ionic charges and employing the electronic continuum correction (implemented through charge scaling) to account effectively for electronic polarisation. The experimental data were then used to assess the successes and shortcomings of the investigated force-fields. We found that due to the very good signal-to-noise ratio in the experimental data, they provide a very narrow window for the position of the first hydration shell of the chloride ion. This allowed us to establish the importance of effectively accounting for electronic polarisation, as well as adjusting the ionic size, for obtaining a force-field which compares quantitatively to the experimental data. The present results emphasise the utility of performing neutron diffraction with isotopic substitution as a powerful tool in gaining insight and examining the validity of force-fields in concentrated electrolyte solutions.     

Structural properties of aqueous electrolyte solutions from i.r. absorption spectra

Summary Results are reported on near-i.r. absorption spectra of H₂O in the presence of a variety of electrolytes, in the temperature range of liquid state. Spectra are analysed by using a deconvolution method, presented and discussed in previous works, thus obtaining a new level of detail and completeness of information. This allows for the first time the separation of solute perturbation on the solvent structure in terms of electric-field effects (common to all solutes), steric effects (mainly determined by the ionic radius) and more subtle effects due to the electronic structure of the specific ion. The quantitative set of spectroscopical parameters obtained for the different solutions in the whole explored temperature range offers a potentially useful ground for comparison with results of computer simulation.

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Riassunto In questo lavoro sono riportati risultati sugli spettri di assorbimento di H₂O nel vicino infrarosso in presenza di una varietà di elettroliti, nell'intervallo di temperatura dello stato liquido. Gli spettri sono analizzati usando un metodo di deconvoluzione, presentato e discusso in precedenti lavori, ottenendone un nuovo grado di risoluzione e completezza d'informazione. Questo ha permesso per la prima volta la separazione della perturbazione indotta dai soluti sulla struttura del solvente in termini di effetti di campo elettrico (comune a tutti gli ioni), effetti sterici (in media dipendenti dai raggi ionici) ed effetti piú fini dovuti alla struttura elettronica dei singoli ioni. Il quadro quantitativo dei parametri spettroscopici ottenuti per le differenti soluzioni nell'intervallo di temperature esplorato offre una base potenzialmente utile per un confronto con i risultati di simulazione al computer.

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Резюме Приводятся результаты для слектров инфракрасного поглошения H₂O в присутствии различных электролитов в области температуп жидкого состояния. Полученные спектры анализируются, используя метод ?развертывания?, предложенный в предыдуших работах с целью получения нового уровня полноты информации. Этот подход впервые позволил разделить возмушение, обусловленное растворенным вешеством на структуру растворителя, на эффекты электрического поля (обшие для всех растворенных вешеств), на сферические эффекты (зависяшие от радиуса иона) и на более тонкие эффекты, обусловленные электронной структурой отдельного иона. Количественная система спектроскопических параметров, полученных для различных растворов во всей исследованной области температур, представляет потенциально полезную основу для сравнения с результатами машинного моделирования.

     

Intermediate range order in concentrated aqueous solutions of copper nitrate. X-ray diffraction and Raman investigations

Concentrated aqueous solutions of copper nitrate of different concentrations (0.66 mol dm⁻³ up to 4.84 mol dm⁻³) have been investigated by X-ray diffraction at room temperature. In these solutions a maximum of intensity (prepeak) is observed at low values of Q (approx. 0.6 to 1 Å⁻¹) suggesting the existence of an intermediate range order in their structure, in agreement with previous investigations on different electrolytes. In order to get information on the solvation shell of the copper cation, Raman spectroscopy experiments have been performed for the more concentrated solution. Polarization observations, isotopic substitution of the solvent and the comparison with a Raman spectrum of a concentrated aqueous solution of aluminum nitrate were performed in order to improve the interpretation of the obtained results with these convergent observations.     

Cooperative diffusion in concentrated polymer solutions

Summary Concentrated solutions of polystyrene in dioctylphthalate have been examined as a function of temperature over the concentration range 0.2 to 0.9 g/ml. The static screening length and the hydrodynamic screening length evaluated from the cooperative-diffusion coefficient are compared with the predictions of current models. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Dynamic properties of aqueous electrolyte solutions from non-polarisable,polarisable, and scaled-charge models

We investigated the dynamic properties of alkali halide solutions (NaCl, NaF, NaBr, NaI, LiCl, and KCl) using molecular dynamics simulations and several non-polarisable, polarisable, and scaled-charge models. The concentration dependence of shear viscosity was obtained with low statistical uncertainties to allow for calculation of the viscosity Jones-Dole B-coefficients. No prior values are available for the B-coefficients from molecular simulations of fully atomistic models for electrolyte solutions. In addition, we obtained diffusion coefficients with rigorous finite-size corrections to access ion mobilities; these provide insights on single ion hydration behaviour. We find that all models studied, even polarisable and scaled-charge models, quantitatively over-predict water structuring but qualitatively follow the experimentally determined Hofmeister series. All ion models considered are kosmotropes based on their calculated B-coefficient and diffusion coefficients, even for ions experimentally found to be chaotropes. These observations indicate that the water-ion interactions in these models are not adequately represented; additional interactions such as charge transfer must be incorporated in future models in order to better represent electrolyte solution properties.     

Viscosity of concentrated polymer solutions

In investigations of the Newtonian viscosity of polymer fluids, two concentration regions have received the major attention. One is the dilute solution where the contributions of the individual solute entities are essentially additive with only small interaction perturbations. The other is the bulk polymer or plasticized melt. We are concerned here with the broad intermediate region.     

Convective boundary layers in electrolyte solutions

During current passage through an electrolyte solution, ions entrain their hydration shells, which gives rise to the accompanying diffusion flow of water molecules and internal convection analogous to that observed in superfluid He³+He⁴ solutions. Hydrodynamic conditions are determined under which the total water flux on a solid surface is zero. The convective boundary layer on a vertical plate is theoretically described using the von Karman method. The diffusion-mechanical effect in thin capillaries is evaluated.